the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Submitted as: model evaluation paper 15 Oct 2021
Submitted as: model evaluation paper | 15 Oct 2021
IBI-CCS: a regional high-resolution model to evaluate western Europe sea level changes
- 1CNRM UMR 3589, Météo-France/CNRS, Toulouse, France
- 2Mercator Ocean International, Ramonville St Agne, France
- 1CNRM UMR 3589, Météo-France/CNRS, Toulouse, France
- 2Mercator Ocean International, Ramonville St Agne, France
Abstract. Projections of coastal sea level (SL) changes are of great interest for coastal risk assessment and decision-making. SL projections are typically produced using global climate models (GCMs) which cannot fully resolve SL changes at the coast due to their coarse resolution and lack of representation of some relevant processes. To overcome these limitations and refine projections at regional scales, GCMs can be dynamically downscaled through the implementation of a high-resolution regional climate model (RCM). In this study, we developed the IBI-CCS regional ocean model based on a 1/12 ° north-eastern Atlantic NEMO ocean model configuration to dynamically downscale CNRM-CM6-1-HR, a GCM with a ¼ ° resolution ocean model component developed for the Coupled Model Intercomparison Project 6th Phase (CMIP6) by the Centre National de Recherches Météorologiques (CNRM). For a more complete representation of processes driving coastal SL changes, tides and atmospheric surface pressure forcing are explicitly resolved in IBI-CCS in addition to the ocean general circulation. To limit the propagation of climate drifts and biases from the GCM into the regional simulations, several corrections are applied to the GCM fields used to force the RCM. The regional simulations are performed over the 1950 to 2100 period for two climate change scenarios (SSP1-2.6 and SSP5-8.5). To validate the dynamical downscaling method, the RCM and GCM simulations are compared to reanalyses and observations over the 1993–2014 period for a selection of ocean variables including SL. Results indicate that large-scale performances of IBI-CCS are better than those of the GCM thanks to the corrections applied to the RCM. Extreme SLs are also satisfactorily represented in the IBI-CCS historical simulation. Comparison of the RCM and GCM 21st century projections show a limited impact of increased resolution (1/4° to 1/12°) on SL changes. Overall, bias corrections have a moderate impact on projected coastal SL changes projections, except in the Mediterranean Sea where GCM biases were substantial.
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Alisée A. Chaigneau et al.
Status: open (until 23 Dec 2021)
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RC1: 'Comment on gmd-2021-328', Anonymous Referee #1, 06 Nov 2021
reply
The authors present the high-resolution regional ocean model IBI-CCS covering a relatively large domain west of Europe, and use it to dynamically downscale the CMIP6 model CNRM-CM6-HR. In the first part of the manuscript, CNRM-CM6-HR, its low-resolution version, and different versions of the downscaling set-up are evaluated mainly against reanalysis data. In the second part of the manuscript, projected changes are compared focusing on sea level, highlighting the effects of dynamical downscaling and a number of corrections that have been applied to the prescribed boundary conditions derived from CNRM-CM6-HR. This has clearly been a major effort, helps to interpret previous downscaling studies for northwestern Europe and offers several new methodological insights. While the model set-up appears to be sound, I have a number of comments on the bias corrections and the evaluation of the model output. Additionally, I think the manuscript is not very well polished. I therefore recommend that the authors check the manuscript to improve the writing (grammar, typo’s, sentence structure) and clarity throughout.
Main comments:
L86 states that the aim of the study is to provide projections of sea-level changes, focusing on methodological aspects. If this is the aim, using only a single (downscaled) GCM is probably insufficient. The specification in L86-L90 suggests, however, that the aim is to evaluate the impact of dynamical downscaling and bias corrections on simulations of sea-level change, while the title suggests that the aim is to ‘evaluate sea-level change’. Other statements in the manuscript imply that the study is meant for presenting and evaluating a regional ocean model that will apparently be used for analyzing extreme sea level in a follow-up study. Altogether, I think that the purpose of the manuscript needs to be more clearly described in the introduction, in the conclusions and possibly in the title as well.
L221-L239: the authors apply a bias correction to the boundary conditions by subtracting the historical mean seasonal cycle of biases, assuming that biases are stationary. However, the seasonal cycle and therefore the associated model biases are likely to change in the future. Could the authors comment on the validity/caveats of their methodology in this light? Showing the size of the bias (corrections) may be insightful as well.
Section 3.1: In multiple comparisons in this section, the IBIRYS and IBI-ERAi products are taken as the ground truth for evaluating the added value of downscaling, while those products rely on models and have their own biases as well. A quantitative comparison against observations to support the case that the changes due to downscaling and bias corrections are actually improvements, seems to be missing.
Section 3.1.6: the authors compare the 99th percentile SSH between IBI-CCS_corr and GESLA2 observations and conclude that their model ‘properly’ reproduces the observed extremes, with an ‘error rarely exceeding 20%’. What is missing, however, is their motivation and contextualization for why errors of 10-20% are acceptable. Additionally, the authors should motivate why assessing only this single aspect of the simulation of ESLs is sufficient evidence that IBI-CCS is ‘a suitable tool (L790)’ for analyzing extreme sea levels and projecting their changes, as the authors seem to plan doing in a follow-up study.
Other comments:
L11: “some relevant processes’ - here and elsewhere, the paper would benefit from the authors more precisely formulating what they mean
L15: suggest changing ‘developed for’ to ‘participating in’
L36: here and throughout, it would be good to avoid complicated compounds such as ‘decision-making processes’ (consider ‘decision making’), ‘SL focus’ (‘focus on sea level’), ‘the model spatial resolution’ (‘the spatial resolution of the model’), SL changes spatial variations (‘spatial variations of SLC’), etc.
L45: remove ‘to’ or replace by ‘due to’
L51: could the authors add an average kilometric resolution? (only few CMIP6 models have a quarter degree resolution)
L56: The authors may consider writing out less commonly used abbreviations like DD, as well as the regional features in Figure 1, as these abbreviations may confuse readers unfamiliar with these terms
L60: ‘thanks to DD’ -> ‘using DD’?
L75: is it dynamical downscaling that can overcome this problem or is it the bias corrections?
L81-82: unclear sentence structure
L85: up to this point the authors have made clear what has been done in the literature, but they have not pointed out what has not been done in the literature and therefore the motivation for and the novelty of their model/study is not yet clear.
L89: the adjective ‘high-resolution’ is somewhat misleading here: the resolution may be high compared to other CMIP6 GCMs, but is not that high compared to some regional ocean models. If it would be, there would be less reason to dynamically downscale.
L89: ‘to these aims’
L99: I suggest to change ‘climate change scenario’ to ‘greenhouse gas concentration scenarios’ and add a reference. Additionally, the choice to use these two scenarios needs motivation.
L104: ‘regional ocean climate model’ change to ‘regional ocean model’?
L103-L106: could the authors clarify why CNRM-CM6-1-HR specifically is chosen?
L111: does this mean that only the horizontal ocean resolution is increased in IBI-CCS compared to the CNRM-CM6-1-HR model?
L119: ‘to CMIP6 typical resolution’ -> ‘to the typical ocean grid resolution of CMIP6 models’
L130: ‘required to force’ -> ‘for’
L135: ‘approximatively’ -> ‘approximately’
L136-137: needs a reference
L149-155: I suggest to change the order: first introduce the grid (resolution) of the IBI-CCS model and then explain its grid is based on an existing reanalysis
L165: perhaps ‘added value’ here should be replaced by ‘expected areas of improvement’ or alike, since the added value is assessed later on in the manuscript
L188: the reason for the different set-up at the eastern boundary is missing
L189: could the authors specify the type of boundary constraints used for the other variables as well?
L190-191: from where are the initial conditions and the runoff derived? From the GCM? Observations? elsewhere?
Section 2.2.1: could this usefully be merged with the text above table 1, leaving just a separate section describing the version with drift/bias corrections?
L211: could the authors specify how large? Does this have any effect on the SSP runs?
L215: please specify which variables, and could the authors explain how they know a linear fit is ‘indeed appropriate’?
L216: subtracted ‘from’
L269: The structure of the manuscript may benefit from a more descriptive title for this section
L270-272: I think this line of reasoning could be clarified by writing that the frequency shift of ESLs depends on total sea-level rise rather than just the ocean dynamic component, hence additional SLC components need to be incorporated in the model.
L273: the contribution of changes in land-water storage to GMSLR seems to be neglected?
Section 2.3.3: it would help to clearly distinguish between sea level and sea-level change in the manuscript. For instance, is defined as total sea level (not sea-level change) but also as the sum of GMSL”R” and SSH. Sentence structure and grammar also need attention in this section.
Figure 3: this figure shows many different things. To clarify things, the authors could consider splitting it up in multiple figures and discussing them one by one. I am also wondering, if the aim here is to evaluate depth-integrated salinity and heat, why these variables are then not evaluated directly, since their biases may be more intuitive to understand than their meter sea level equivalents. Also, in all maps in the manuscript, it would be helpful to indicate some of the relevant isobaths in the regions (e.g., the shelf break).
Figure 4: apart from panel c, the arrows depicting the currents are too small to recognize
Figure 6: it would be helpful to mention that (presumably?) all panels here do not include the inverse barometer effect on mean sea level (same for Fig7?). Additionally, can the authors point to any reason for the differences in MDT between (a) and (b), especially in the Celtic Sea?
L462: the authors mention that all models reproduce the observed MDT (6a) ‘well’. This assessment needs further justification given the differences between modelled and observed MDT in Figure 6.
L572-574: can the authors explain why it is expected that IBI-CCS_corr matches better with projections from CMIP(5?) models than IBI-CCS_raw? One may expect this to be the other way around, assuming that the CMIP models referred to also have deficiencies in resolving the Mediterranean Sea?
Figure 12: can the authors explain why they only computed steric changes 0-2000 m, and what the implications are for comparing results against studies that have integrated down to the full depth of the ocean? Does this mean that manometric + steric change in this case is not equal to total DSLC, since steric changes below 2000 m depth are missing? If so, that should be mentioned. Also, is the change of the inverse barometer effect incorporated in the manometric change here?
L593-611: I do not fully understand the sign change of thermosteric SLC in large parts of the Atlantic between SSP1-2.6 and SSP5-8.5. Additionally, the positive halosteric SLC in the Atlantic implies the whole 0-2000 m is getting fresher everywhere in the deep ocean. Can the authors point to any reasons for these effects? Have columns c and d of Figure 12 been interchanged accidentally, by any chance?
Figures 14 & 15: the white dots do not strongly contrast the shading of the figures, and in many cases cover most of the domain. I suggest choosing another color and only stippling those parts that have insignificant differences.
L634: some caution is needed in attributing all of these differences solely to resolution issues, since resolution is not the only aspect in which the models differ.
L668-669: the authors state that the sea-level projections of IBI-CCS_raw and IBI-C’S’S_corr (note the typo) are very similar. However, this seems to be contradicted by Figures 15b-d, which show fairly large differences in the deep ocean. Since the differences in DSLC (Figure 15a) are smaller, apparently manometric SLC also differs substantially due to the bias corrections. Could the authors comment on this and explain why the bias corrections appear to affect the steric and manometric changes in the deep ocean in a substantial but compensating manner?
L689: significant differences or significant changes? These plots do not show differences between models do they?
L700-702: could the authors point to any potential physical mechanism behind the decrease of the magnitude of interannual sea-level variability in the North Sea?
L749-750: it would be good to refer to Hermans et al. (2020) here once more, since they already argued the same thing, based on downscaling two GCMs differing in ocean grid resolution in the region
L751-752: it would be good to add that to verify this, CNRM-CM6-1 would need to be downscaled as well
L776-777: Other GCMs may have other biases that may require different corrections. Different types of corrections, for example based on different reanalysis products, have not been tested here. Additionally, the model set-up in this manuscript requires more computational effort than time-slice methods such as used by Jin et al. (2021), which makes it difficult to obtain the large ensembles that are eventually required for comprehensive projections. Some more discussion of the caveats and the wider application of the methods in this study seems warranted.
Finally, the authors may consider adding another test, comparing a simulation with and without tides, for example in terms of seasonal biases and the simulation of mean sea-level change. This may be well outside the scope of the manuscript, but if relatively easily implemented in the IBI-CCS framework, it could give valuable insights into the limitations of models excluding tides in the context of projecting mean sea-level change.
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RC2: 'Comment on gmd-2021-328', Anonymous Referee #2, 03 Dec 2021
reply
The manuscript entitled “IBI-CCS: a regional high-resolution model to evaluate western Europe sea-level changes” by Chaigneau et al.. deals with the sea level projection for the north-eastern Atlantic region via a dynamical downscaling of a set of CMIP6, single model, simulations. The scenarios considered are the SSP5-8.5 and SSP1-2.6 The manuscript, however, is mainly focused on assessing the impact on the modelled regional sea-level change of (i) increased horizontal resolution (ii) a complete representation of coastal processes, (iii) application of a bias correction to the driving GCM.
Main concerns
The title of the manuscript is misleading. This manuscript mainly focuses on assessing the impact on the modelled regional sea-level change of an increased horizontal resolution of the DD, a more complete representation of coastal processes, and applying a bias correction to the driving GCM. Using only a single GCM downscale is not sufficient to provide a reliable sea-level change estimation. I suggest changing the title of the manuscript to reflect the primary goal of the work.
Paragraph 2.1.2 Regional ocean model IBI-CCS: Tide in the regional configuration is one of the main processes driving SL change in coastal areas. A specific validation of the tides should be included in the manuscript.A reference to a peer-reviewed paper in which the tides have been validated is also enough. Moreover, the authors claim that “Tides are included in the model by calculating the astronomical tidal potential and the tidal harmonic forcing as …”. Here the author should be more clear on the way they applied the tidal forcing in the regional model.
Paragraph 2.3.2. In line 308, the authors state that “The GCM GMTSLR term stored in the variable “zostoga” is thus added a posteriori to the RCM modelled SL”. My deep concern is how the authors used this variable in the final SL computation (Figure 9). The global model used in this study is affected by strong temperature (and salinity) drift due to its relatively short spin-up (250 yrs). In particular, the temperature drift affects the local thermosteric component of the SL, and so the global mean thermosteric component (zostoga). Maybe I am wrong, but it seems that the authors used the original zostoga variable provided by the global simulation without any correction. I suggest the authors to indicate in the manuscript how they treated zostoga before using it in the final SL computation.
Line 172: I suspect that mixing due to internal tides is overestimated. So, I suggest to provide more details about the de Lavergne scheme and a more robust justification about its use in the regional model.
Line 249: Sea Surface Height tuning in the Mediterranean Sea. The authors claim that “GLORYS2V4 has a mean SSH bias of approximately -0.1 m in the Mediterranean Sea in comparison to the Mean Dynamic Topography observations from CNES- CLS-18”. It would be good to show, at least in the supplementary material, the horizontal map showing the differences between GLORYS2V4 and CNES- CLS-18 over the entire domain. The -0.1 value used as a correction seems to result from a tuning exercise. The authors should provide more details on the applied correction if this is the case. Also, looking at Figure 3, it appears that in all simulations (including IBI-CCS_corr) there is a bias on the eastern boundary in the Mediterranean Sea. Do the authors have a valid justification for the bias in IBI-CCS_corr?
As a general comment, the manuscript needs revision for language and grammar.
Minor issues
Line 13: Please, include the name of the model “(Iberian-Biscay-Ireland Climate Change Scenarios)”
Line 63: Please, provide the physical definition for “dynamic sea level”.
Line 75: The authors claim that “The DD method can be used to overcome this problem by applying corrections to the GCM outputs before using them as forcing when performing a DD”. Actually the bias in GCM simulations can be strongly reduced using bias correction. So, I do not agree in ‘the DD methods’. May be this sentence need to be revised or deleted.
Line 114: Paragraph 2.1.1. It would be good to add a specific subparagraph in which is indicated how the SSH is modelled in both global models.
Line 115: I did not find any specific paper in the literature dedicated to the validation of CNRM-CM6-1-HR. Am I wrong? In case you could not provide any reference to the validation of CNRM-CM6-1-HR it would be necessary
to justify the use of this model simulation as driver for the DD.
Line 122: The following sentence is not clear to me: “A polynomial representation of the equation of state (TEOS-10, Roquet et al., 2015) is used but the temperature and salinity outputs are converted into the in-situ temperature and practical salinity needed by the RCM. “. Please, rewrite this sentence.
Line 128: please, insert citation for OASIS-MCT
Line 129 please, insert a citation for ARPEGE-Climat 6.3
Line 130: I would suggest to indicate the exact number of simulations used.
Line 138: In this paragraph should be indicated the model resolution of the regional model
Line 151: I don’t think it is relevant to indicate the 1/36° version in the manuscript. It is not used for validation. So, I suggest to remove it.
Line 152: Please, provide the explicit link in the references.
Line 230: Please, provide the link in the references
Line 240: The paragraph is not enough clear. Please, rewrite this paragraph.
Alisée A. Chaigneau et al.
Alisée A. Chaigneau et al.
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